Mice were evaluated with respect to (a) their ability to give an antibody response to an optimally immunogenic dose of Type III pneumococcal polysaccharide (SSS-III), (b) the degree of suppressor T cell activity, and (c) the degree of amplifier T cell activity present at different ages after birth. The ability to respond to SSS-III increased rapidly with age, reaching adult levels at 4 weeks of age; the response of 2 and 3 week-old mice was 7 percent and 40 percent, respectively, of that found in adult (8 week-old) mice. Suppressor T cell activity was evident at least as early as 2 weeks of age and was just as profound as that of adult mice. However, amplifier T cell activity developed slowly and did not achieve adult levels until 8-10 weeks of age. The dominance of suppressor T cell activity in young mice suggests that their inhibitory effects may contribute to the ease with which tolerance can be induced in neonates, and prevent the development of auto-immune responses.